Potassium monopersulfate (KHSO5), also known as potassium peroxymonosulfate, is a component of a triple salt with the formula 2 KHSO5—KHSO4—K2SO4. Due to the high oxidation potential of potassium monopersulfate (“PMPS”), the PMPS triple salt 2 KHSO5—KHSO4—K2SO4 makes a good candidate as a component in bleaches, cleansing agents, detergents, and etching agents, and also as an oxidizing agent in inorganic reactions.
While PMPS's strong oxidation potential is well known, PMPS is limited in its utility because of the presence of an irritating byproduct, K2S2O8. The severe irritating qualities of K2S2O8 and its inherent stability relative to the desirable KHSO5 limit the use of PMPS to products that would not come in contact with its users. Thus, while PMPS could be used in personal care products, manufacturers do not use PMPS for the fear that users of these products will experience irritation from the K2S2O8. The irritating effects of K2S2O8 even limit the use of PMPS in products that come into contact with users (and their pets) indirectly, such as surface cleaners, laundry bleaching agents, and swimming pool water treatment solutions. Even low levels of K2S2O8 accumulated in pool water or laundry as residues can cause undesirable effects on humans and pets that come into contact with it. Ideally, to be able to use PMPS in these products, the level of K2S2O8 as a byproduct should be <0.1 wt. % of the PMPS.
One way to reduce or eliminate the fraction of K2S2O8 in a PMPS product is to increase the yield and stability of the desirable KHSO5 without using oleum, since the use of oleum results in the production of K2S2O8. Since a higher active oxygen content in the end product correlates with a higher fraction of KHSO5, it is desirable to achieve a PMPS composition with increased active oxygen content and higher stability using H2SO4. Publicly available Caro's acid conversion data (e.g., data from FMC Corporation) indicates that with H2SO4 to H2O2 molar ratios of 1:1 and 2:1, the active oxygen obtained from the Caro's acid equilibrium products yields 4.3% and 3.7%, respectively.
Typically, PMPS triple salt is produced by using Caro's acid (H2SO5, also called peroxymonosulphuric acid). Caro's acid is usually produced by reacting H2SO4 or oleum with H2O2. More specifically, Caro's acid is an equilibrium product between these reactants on one hand and H2SO5 and H2O on the other, as shown by the following reaction:H2SO4+H2O2<<>>H2SO5 (Caro's acid)+H2O.As the molar ratio of H2SO4 to H2O2 increases, the yield of H2SO5 increases. Thus, in order to optimize the amount of Caro's acid that is produced, excess H2SO4 or oleum is added during the process.
The Caro's acid is reacted with alkali potassium salts such as KHCO3, K2CO3, and/or KOH to generate KHSO5:H2SO5+KOH→KHSO5+H2O.Thus, increasing the yield of Caro's acid results in a higher concentration of KHSO5, which helps reduce formation of the irritant K2S2O8. The potassium to sulfur ratio (K/S) is controlled to produce a specific composition. Generally, a K/S of <1.0 will result in a high yield of KHSO5 because K/S>1.0 induces some attrition of the desired salt to produce K2SO4.
However, the salt resulting from K/S<1.0 is too unstable for most commercial applications and is hygroscopic (absorbs water). To make a stable, nonhygroscopic triple salt, a sufficient level of K/S must be achieved to produce the stabilizing sulfate salts (i.e., KHSO4 and K2SO4). In producing these compositions, the excess potassium (K/S>1.0) reacts with both KHSO5 and KHSO4, following an attrition close to their molar ratios. The decomposition of monopersulfate reduces the A.O. level in the resulting triple salt and increases sulfates.
Various parameters have been manipulated to optimize Caro's acid production. One of these parameters is reaction temperature. Temperature has been controlled to reduce the decomposition of Caro's acid, which results in release of oxygen and increase in sulfate salts, neither of which is desirable. Some knowledge regarding preparation of Caro's acid and PMPS triple salt are provided in the following references:                U.S. Pat. No. 3,939,072 (“the '072 patent”) teaches a process for point of use production of Caro's acid, in which the Caro's acid is cooled to between −10° C. to 80° C. to reduce decomposition of the Caro's acid before its use.        U.S. Pat. No. 5,141,731 (“the '731 patent”) teaches a process and an apparatus for point of use generation of peroxyacids by adding H2O2 to a stream of H2SO4 in multiple stages. The H2SO4 is cooled to between 15 to −40° C. before this addition. After the addition, the resulting solution is cooled to a temperature of 0 to 80° C. to reduce the decomposition of Caro's acid. The Caro's acid has to be diluted with water or used immediately thereafter, before decomposition of the Caro's acid happens. As in the '072 patent, the cool temperature is maintained to prevent A.O. loss that is generally caused by a higher temperature that results from the exothermic reaction. The resulting solution is reported to be 15% higher in H2SO5 when using multiple additions of H2O2 versus one addition. However, if the dilution with water or the use of the Caro's acid is not immediately done after the H2O2 addition, the equilibrium reaction takes place and the A.O. level rises to about 4.3. In lab experiments, Caro's acid solution is produced over a period of about 20 seconds, diluted with water to a solution strength of less than 200 g/l to stop the reactions, then chilled to preserve the Caro's acid for analysis. In practical use, the invention requires a series of stages wherein some amount of H2O2 is added to the oxyacid in each stage, mixed, and cooled.        
This method illustrates that a higher percentage of H2O2 conversion can be achieved by controlling the order of addition of the reagents. However, the resulting Caro's acid solution must be used immediately after production as is the case utilizing the disclosed invention, or rapidly diluted with water in order to preserve the benefits of the invention. If not used or diluted immediately after its production, as disclosed in literature and prior art, the KHSO5 portion of the Caro's acid solution will decompose to achieve the equilibrium product that is well established in the prior art, resulting in a triple salt having an A.O. of ≦4.3.
Another shortcoming of this method is that it is difficult to implement with the use of traditional single-stage reactors. This technique requires multiple series of reactors, each independent of the other, to provide a single pass process. Naturally, this process excludes the use of traditional single-stage reactors such as batch or stirred tank reactors since addition of the H2O2 requires substantially more time to complete the addition and reaction before application or dilution whereby the reactions, including the equilibrium reaction, are sequestered.                U.S. Pat. No. 5,429,812 (“the '812 patent”), which discloses a process of producing peroxysulfuric acid from substoichiometric levels of H2SO4 to H2O2, teaches using a substoichiometric amount of H2SO4 to produce an equilibrium amount of Caro's acid. The final mixture in the '812 patent has a molar ratio of SO3 to Available Oxygen in the range of 0.8 to 0.2. The '812 patent also teaches that the order in which these reagents are introduced does not affect the Caro's acid yield. The reagents used were 70% H2O2 and 93% H2SO4. The '812 patent discloses that regardless of taking steps to avoid decomposition such as cooling and agitation, trials demonstrated that equilibrium occurred very quickly when the reactants were brought into contact, and that the position of the equilibrium depended consistently on the molar concentrations of the reactants, independently of the order of introduction.        
As disclosed in the '812 patent, even with adequate cooling and agitation to prevent decomposition, the equilibrium proceeds rapidly and results with an A.O. value consistent with the established equilibrium products. This occurred regardless of the order of reactant addition and was independent of the reactant concentrations, which include H2O concentration. Also, previously, it was known that using 70% H2O2 and H2SO4 will result in a Caro's acid solution with an active oxygen content of no greater than 4.3% at a 1:1 molar ratio.                U.S. Pat. No. 5,139,763 (“the '763 patent”) teaches making Caro's acid with a supra-stoichiometric molar amounts of oleum to H2O2. It discourages using H2SO4 on the grounds that a higher molar equivalent of H2SO4 is required to obtain similar yields of H2SO5 compared to oleum, resulting in a higher manufacturing cost. Also, when this high molar equivalent of H2SO4 is used, the molar ratio of the resultant solution has a H2SO5 to H2SO4 ratio that is less than what is desired for the preparation of the PMPS triple salt. The Caro's acid is partially neutralized to raise the K/S to 1.15–1.25, then combined with a solution richer in monopersulfate.        
The method of the '763 patent involves many steps and results in an undesirably high concentration of K2S2O8.                U.S. Pat. No. 5,607,656 (“the 656 Patent”) describes a process for producing PMPS with high available oxygen and a low concentration of K2S2O8. This process involves reacting 20 to 70 wt. % strength oleum with 30 to 70 wt. % strength hydrogen peroxide to form Caro's acid, partially neutralizing the Caro's acid, then adding sulfuric acid and potassium hydroxide to the mixture by injection into the vacuum crystallizer while evaporating off the moisture. The resulting wet salt has a K2S2O8 concentration of less than 1.5 wt. %, which is reported to be less than that found in the commercially available triple salt. However, the commercial advantage of this process is limited by the increase in cost associated with all the additional reagents (higher SO4 to H2O2 molar ratio) required to dilute the K2S2O8 concentration in the triple salt, and the resulting A.O. as compared to the initial Caro's acid solution.        
The '656 patent discloses a process for producing a triple salt with reduced oxodisulfate by reacting Caro's acid produced from oleum with additional H2SO4 and KOH. This dilution process utilizes established processing techniques as previously disclosed. Like other disclosures, the critical chemistry and control parameters are met to produce the resulting triple salt.                U.S. Pat. No. 4,579,725 (“the '725 patent”) describes a process for producing PMPS with high available oxygen and low K2S2O8 by partially neutralizing the Caro's acid produced from 65–75% oleum and H2O2 by reacting the reagents at a sulfur to peroxide molar ratio of 0.9 to 1.2. The Caro's acid is reacted with KOH to achieve a K/S ratio <0.95. The resulting slurry is concentrated by using vacuum evaporation so that the fraction of the slurry solids is sustained at <40%. The mother liquor that is rich in KHSO5 is recycled back to the evaporator. MgCO3 is aggressively added to the concentrated slurry to control the K/S ratio to yield a product of high A.O. The MgCO3 treatment is needed because the product has low-K/S product has low stability and melting point.        
The '725 patent uses 65–75% oleum to produce Caro's acid, performs partial neutralization with KOH solution to achieve K/S ratio <0.95, concentrates using vacuum evaporation to slurry solids of <40%, forms a wet cake while returning concentrate back to the evaporator, adds MgCO3 to the cake, mixes and dries, and adds more MgCO3.
The resulting monopersulfate salt from the low K/S ratio is hygroscopic and unstable. Coating with MgCO3 was shown to stabilize the salt. MgCO3 has been used as an anti-caking agent to improve fluidity of the triple salt for many years.                U.S. Pat. No. 4,610,865 (“the 865 Patent”) discloses a process to produce and concentrate a solution containing KHSO5 to a monopersulfate concentration of 20–30 wt. % KHSO5, cooling a partial stream to <15° C. to precipitate the triple salt, filtering the triple salt, and drying.        
Like the '725 patent, the '865 patent defines specific chemical and control parameters like those disclosed in the expired prior art patents mentioned above, to produce a composition of triple salt precipitated from a solution of KHSO5 using a cold precipitation technique. The equipment and methods of producing the Caro's acid, triple salt, concentrating and separating are consistent with previously disclosed methods of processing.
The resulting monopersulfate, like that in the '725 patent, is produced from substoichiometric levels (excess sulfuric acid) of potassium to sulfur, and therefore is hygroscopic and exhibits poor shelf life.
All of the disclosed methods of producing a stable, non-hygroscopic (K/S>1.15) triple salt of reduced K2S2O8 with high active oxygen (>4.7%) require additional treatment of the slurry streams, reprocessing of solutions of triple salt to dilute the K2S2O8, and/or additional treatment steps to increase stability and melting point of the resulting triple salt. In doing so, waste streams of discarded inert salts such as K2SO4, and/or multiple processing steps, high recycle rates, and elaborate process control scenarios are proposed.
Because of the indirect nature of producing these hybrid triple salts, their commercial viability is severely impaired due to the increased production cost resulting from product waste (discarded salts) and/or extensive recycling and reprocessing of the triple salt solutions.
Thus, the search for a way to efficiently produce PMPS triple salt with less irritant byproducts (e.g., K2S2O8) and higher active oxygen with a high stability at a reasonable cost continues.